A Digital Subscriber Line (DSL) is a technology that effects digital communication to customers over an existing line, typically a twisted-pair in a wire cable comprising the telephone loop plant. The current loop plant environment, including bridged taps and mixed metallic wire gauges, was originally designed for voice frequency transmission. Now, however, this loop plant presents a complex environment for wideband transmission such as digital data services. In order to economically provide wideband services, the DSL must be implemented without conditioning the loop plant (e.g., by removing bridged taps or by rearranging pairs), notwithstanding the detrimental effects of bridged taps and gauge changes. Furthermore, no special engineering or operations can be associated with DSL installation.
"Impulse noise" is one of the most difficult transmission impairments to suppress in a DSL environment. Impulse noise is noise that appears on the line during short intervals that are random in their occurrence. Impulse noise has many causes and there are no universally accepted explanations for its appearance or models of it. There are many suspected causes for impulse noise, such as:
(1) Longitudinal transients that result from relay closures at central office terminations. The longitudinal transients easily couple between pairs and then between the longitudinal and metallic circuits of a given pair because of imbalance to ground in the pair. PA1 (2) Telephone sets going off-and-on hook at the station end. The coupling mechanism is substantially the same as in the case of relay closures. PA1 (3) Powerful electrical equipment, which is connected to a power line that runs along a telephone cable, switching on-and-off. Again, coupling is initially between longitudinal circuits and then through longitudinal-metallic imbalance in the disturbed pair. PA1 (4) Poorly grounded equipment that is attached to the telephone network on leased and private lines. Coupling is directly between metallic circuit and longitudinal circuit of the disturbing pair. PA1 (5) Craft activity in the repairing and/or rearranging of telephone cables. PA1 (6) Lightning. PA1 (a) an impulse may have much greater energy than the signal segment that it is impressed upon; PA1 (b) the time of arrival of an impulse is unpredictable--even the probability distribution of inter-arrival times is uncertain; PA1 (c) because of the impulsive nature of the noise, time-invariant filtering does not work especially well; PA1 (d) the shapes of the impulses tend to be quite varied and relatively little is known about impulse shapes; and PA1 (e) the characteristics of impulses change from location to location and they are evolving as equipment both connected to and influencing the telephone network evolves.
There have been many surveys of impulse noise, but almost all of these have studied impulse noise that was confined to the low tens of kilohertz. The classic approach to combating low-frequency impulse noise was to provide a signal powerful enough to render impulse noise relatively inconsequential. This often proved futile because impulse noise has such high energy density for short intervals.
More recently, with the advent of Integrated Digital Services Network (ISDN), High Rate Digital Subscriber Line (HDSL), and Asymmetrical Digital Subscriber Line (ADSL), there has been interest in impulse noise at higher frequencies. With wider-band impulse noise, research and concomitant field measurements have focused on the shape and spectra of the impulses. As a result of what has been generally learned, error correcting codes have been introduced to overcome some of the deleterious properties of impulse noise in high-frequency communication systems. Those important properties of impulse noise at high frequencies, in contrast to its causes, include:
These characteristics certainly make impulse noise a formidable impairment. However, recent data from a field study of impulse noise indicates that most impulses on a given pair have approximately the same shape. It is certainly reasonable to believe that any impulse on a given line could have one of a relatively small number of shapes. Presumably, the causes of impulses on a given line are largely recurring events--for example, from above, closure of a line relay on an adjacent cable pair.
If there are only a few impulse shapes on a given pair, then adaptive pattern recognition techniques might be used to determine the shapes of the impulses and then the knowledge of the shapes used to cancel the impulses when they occur. One of the problems with this is deciding exactly when an impulse occurred and deciding which of the shapes has occurred.
But presently there is no teaching in the art whereby impulse noise can be recognized, located and canceled as the impulses occur.